Electrolytic preparation of poly-p-xlylenes



United States Patent 3,399,124 ELECTROLYTIC PREPARATION OFPOLY-p-XYLYLENES. Heinrich G. Gilch, Plainfield, N.J., assignor to UnionCarbide Corporation, a corporation of New York No Drawing. Filed Sept.17, 1964, Ser. No. 397,316

30 Claims. (Cl. 204-72) This invention relates to a process for thepreparation of poly-p-xylylenes. More particularly, this inventionrelates to the electrolytic preparation of poly-p-xylylenes.

Heretofore, the preparation of poly-p-Xylylcnes has been accomplished,generally, by pyrolytic processes. These processes have not been highlysuccessful, economical or commercially feasible since they are complexand the amount of poly-p-xylylenes obtained therefrom have beenrelatively small and of little commercial value. Furthermore, theseprocesses employ relatively high temperatures and involve interferingside reactions which render them difiicult to control and commerciallyunacceptable.

It is an object of this invention, therefore, to provide a process forthe preparation of poly-p-xylylenes which is simple, economic andcommercially feasible.

Another object of this invention is to provide a process which does notrequire the use of high temperatures and avoids significant loss ofdesired end product as a result of interfering side reactions. 5

A further object of this invention is to provide a process which isfast, clean and results in higher operational efliciency than has beenheretofore realizable.

Now, in accordance with the present invention, poly-pxylylenes can beconveniently and readily obtained by the electrolysis of ot-haloxylenecompounds having the general structure wherein Y is a member selectedfrom the group consisting of hydrogen and halogens; X is a halogenhaving an equal or lower bond strength than that of Y with the provisothat when Y is fluorine, X is a halogen having a lower bond strengththan fluorine; z is an integer from 0 to 4; R is an aromatic nuclearsubstituent group such as alkyl, aryl, alkenyl, amino, cyano carboxyl,alkoxy, hydroxy alkyl, carbalkoxy and inorganic radicals such ashydroxyl, nitro, halogens and other groups which are normallysubstitutable on aromatic nuclei.

As employed herein the term bond strength is understood to mean thatquantity of energy required to break the covalent bond existing betweenthe carbon atom and the substituent employed in the a-POSitiOnS of the.starting compounds. 1

The ot-haloxylene compounds selected as the starting materials andhaving the general structure wherein Y, R, X and z are as above, aremixed in an electrolytic solvent and placed in an electrolytic cellequipped with a suitable anode and cathode.

An electromotive potential, supplied from any appropriate externalelectrical source, is applied across the electrodes. The amount ofpoly-p-xylylenes which can be obtained from the present process, asmeasured by the current efiiciency, is in an amount of between about60%-95% and higher.

The term current eificiency is understood to mean that quantity ofpoly-p-xylylene obtained as determined by the following equation:

XYzC- OYgX 26 v wherein X, Y, R and z are as above; e represents anelectron and n can be an integer of between about 50-50 and higher.During electrolysis, the X- (halide) is oxidized to the halogen whichreaction is represented by the equation:

wherein X and e are as above.

By employing an a-haloxylene compound having the general structurewherein X, Y, R and z are as above, it can be readily seen that a greatvariety of compounds can be selected for use as the starting material inthe instant process. Typical of such-compounds which can be employed aresuch oc-haloxylenes as Surprisingly, polymerization of the u-haloxylenesoccurs even though neither functional groups such as ethers, esters,amides, and the like, nor functionally active sites such as olefinicunsaturation are present in the a-haloxylenes employed.

The electrolytic solvent employed in the present invention can becomprised of any solvent system capable of acting as a good conductorwhile remaining inert during the reactions which take place in thesystem and also inert to the reaction product obtained.

It has been found that when only water is employed as the electrolyte,hydrogen formation occurs interfering with the electrolysis andpreventing electrolytic reduction of the u-haloxylenes.

It has also been found that the water-soluble organic solvents can notbe used alone as the electrolyte since they have little or no conductiveproperties.

Hence, suitable solvent systems have been found to be mixutres of waterand water-soluble organic solvents such as tetrahydrofuran and water,dioxane and water, dimethylformamide and water, and so forth.

It has also been found that the conductivity of the solvent system canbe materially increased and strengthened when a compound capable ofdissociating into ions 3 in the solvent system such astetramethylammonium chloride, lithium chloride or a small amount ofstrong mineral acid, such as concentrated hydrochloric acid,concentrated sulfuric acid, concentrated nitric acid and so forth isadded to the solvent in an amount of between about 2%6% by volume of thesolvent. A particularly suitable solvent system which can be employed inthe present invention is that which consists of a mixture of dioxane,water and about 2% by volume of concentrated hydrochloric acid.

The anodes employed in the present process are selected from thosematerials which will not enter into reaction with any free halogensreleased during the process. Typical materials which can be employed areplatinum, platinum metals, carbon and the like. In order to assure thatthe halogen formed at the anode does not enter into reaction with thereaction product, the anode can be isolated from the reaction bysurrounding it with a suitable diaphragm so that the liquid surroundingthe cathode is separated from the liquid which surrounds the anode.

If the halogen gases which form at the anode contact the liquidsurrounding the cathode or the cathode itself, interfering sidereactions will take place which will lower the efliciency of the systemand decrease the amount of polymer obtainable. Additionally, there mayalso result a reversible reaction whereby the starting compound isobtained instead of the desired poly-pxylylene.

The diaphragms which can be employed to isolate the anode should beselected from those materials which are porous, non-conductors, andwhich will not react with the electrolyte. Generally, any porous,ceramic material can be employed for this purpose such as silicates,aluminates, sintered aluminum oxides and so forth.

It is significant in the practice of this invention that when aqueouselectrolytes are used, the material selected as the cathode be such thatit will not affect the electrolysis of the solvent system whenrelatively low voltages are employed. If the electrolytic solventcontains any water or acid, the cathode must be capable of exhibiting anovervoltage sutficient to suppress the formation of hydrogen at thecathode.

The term overvoltage as employed herein is understood to mean thatdifference between the potential necessary to form hydrogen at thecathode and the theoretical potential. Hence, any material can be employed for the cathode which will not electrolyze the solvent system andwhich will not produce hydrogen before the u-haloxylene compounds arereduced. Among the materials which can be used for the cathode aremercury, lead, cadmium, zinc, aluminum and so forth.

The overvoltage requirement of the cathode in aqueous electrolyticsolvent systems gives rise to the proviso in the a-haloxylene startingcompounds that when Y is fluorine, X must be a member selected from thegroup of halogens having a lower bond strength than fluorine. It hasbeen found that when X and Y are both fluorine in the a-hZIlOXYlCIlBcompound, the formation of hydrogen at the cathode will occur before thea-haloxylene compound employed is reduced. This phenomenon is confinedonly to the condition wherein Y is fluorine in the a-haloxylenecompound. When Y is any other halogen, this phenomenon is non-existentand the condition for the proviso with regard to X is no longernecessary. Hence, when Y is any halogen other than fluorine; such aschlorine, bromine or iodine, X can be the same halogen or a halogenhaving a lower bond strength than Y. For example, if Y were bromine, Xcould also be bromine, or, if desired, iodine.

The voltage applied to the system should be sufficient to polymerize theot-haloxylenes employed as starting compounds but lower than thedecomposition voltage of the electrolytic solvent system. Generally, thevoltage applied across the electrodes can be between about 3-40 volts.The variation in the voltage required is reflected by and dependent uponthe geometry and size of the electrolytic cell employed, the solventsystem selected, and the ability of the starting material to be reduced.For optimum results, the cathode potential, as measured against aCalomel electrode placed in the system, should not exceed about 2.0volts. However, depending upon the reduction-oxidation potential of thea-haloxylene compound employed, lower cathode potentials can berealized. For example, a,a,a,a,a,a'-hexachloro-p-xylene was polymerizedat a cathode potential of about O.7 volt andu,a-dibromo-a,e,a',aftetrauoro-p-xylene was polymerized at a cathodepotential of 1.10 volts.

When cathode potentials are employed which are significantly greaterthan the reduction-oxidation potential of the a-haloxylene compoundused, interfering side reactions take place which result in poor polymeryield or in the production of polymers containing undesirable stilbeneunits.

The rate of polymerization of the a-haloxylene compounds is directlyalfected by the current in the system. The conductivity, in turn,depends upon the temperature of the electrolyte, the nature and type ofcathode employed and the electrolytic solvent system selected. Hence,polymeriaztion rates can be closely controlled by adjusting the currentin the system. The means by which this can be accomplished are wellknown to those skilled in the art.

The poly-p-xylylenes obtained collect on the cathode and can be readilyrecovered therefrom in particulate form either continuously, as byfiltration, or as a film by any suitable mechanical means.Alternatively, the electrolyte can be stirred and the polymer recoveredtherefrom by subsequent filtration. Hence, the polymeric end product canbe recovered in desirable forms; such as, a fusible polymer, a film, acoating or as the stable, inter mediate p-xylylene as when the processis conducted at low temperatures as set forth in detail hereinbelow.

The polymers obtained in accordance with the present invention have beenfound to "be desirable for use in films, surface coatings, electricalinsulation and other similar applications and have been found to beparticularly useful where high resistance to thermal and chemicaldeterioration are required.

The following examples are set forth as being illustrative of thepresent invention and are not intended to be limitative thereof. Unlessotherwise specified, all percentages and parts are by weight.

Example I A platinum anode and a mercury cathode were placed in anelectrolytic cell containing an electrolyte consisting of 50 ml. ofdioxane, 10 ml. of water and 2.0 ml. of concentrated hydrochloric acid.2.0 grams of u,a'-Cli bromo-u,a,a,u'-tetrafluoro-p-xylene, which can beobtained by reacting terephthaldehyde with sulfur tetrafluoride followedby bromination with N-bromo-succinimide according to the Wohl-Zieglerprocess, was dissolved in the electrolytic solvent. The cathodepotential was kept at about 1.10 volts as measured by a Calomelelectrode placed in the system by using a power source that produced aconstant potential. The voltage between the anode and cathode fluctuatedbetween about 4.9 volts-6.0 volts. The electric current in the systemincreased from about 60 milliamperes at the beginning of theelectrolysis to about milliampercs at the end of the electrolysis.

After an interval of 2 hours 30 minutes, 819 coulombs had passed throughthe electrolyte at which time electrolysis was interrupted.

The polymer was recovered from the solvent by filtration, washed withdioxane and dried at 120 C. under reduced pressure for 15 hours.

The amount of polymer recovered was 0.7716 gram corresponding to acurrent efficiency of 95%. Infrared spectrum analysis of the polymerobtained showed it to be consistent with the structure ofpoly(ot,a,ot',ot'-tetrafiuorop-xylylene). Elemental analysis of theproduct as calcu- 6 p-xylylenes can be obtained utilizing the process ofthe present invention is illustrated by the following example.

Example VIII lated for poly(a,a,a',ct'-tetrafiuoro-p-xylylene) was as Aplatinum anode and a mercury cathode were placed follows: in anelectrolytic cell containing an electrolyte consisting Calculated:54.54% C; 2,27% H; 43 18% F, P d; of 100 ml. tetrahydrofuran, ml. waterand 4.0 ml. of 54,25 C; 2,56% H; 43 11% F, 1 concentrated hydrochloricacid. The cathode was isolated from the remainder of the system by meansof a ceramic Example II diaphragm. A 2.0 gram sample ofDt,Ot,Ot,OL',OL',Ot'-hCXaChlO- 1O ro-p-xylene, which was commerciallyobtained, was dis- III the Same manna! as 15 set forth In EXamPIa 1above, solved in the electrolytic solvent whose temperature wasot-haloxylenes containing nuclear aliphatic, aromatic and h lowered toabout 10 The cathode potential y g p also be p y to Obtain the was keptat about 0.7 volt, as measured by a Calomel respohdlhg p y-py y ThatWhan Such electrode placed in the system, by employing a power oxylenesas y y source that provided a constant potential. The voltage be- 'P Y'P- Y o Y P y are tween the anode and cathode fluctuated between about3.5 used as the stal'hhg compounds, the respechvely resulting volts-4.5volts. The current in the system increased from p y are P y( -py y p -pabout 30 milliamperes at the beginning of the electrolysis P' Y Y and Py( y'p' y y These P y- 90 to about 90 milliamperes at the end of theelectrolysis. mers are obtained y utilizing Process of Example I Afteran interval of 62 minutes, the electrolysis was in- Wherein the cathodepotential 18 bfitWeeIl about 1.0 to terrupted The catholyte that is theS111. -1.5 volts, the voltage between the anode and the cathode roundedthe isolated cathode, was removed f o the is between 3.0-5.0 volts andthe current is between about tem and concentrated at a temperature fabout 2 5-90'I11i11iamPeIS- Uhder these conditions; a current chal- C.by vacuum distillation. The concentrated catholyte was 3 of hetweahabout 75 %-80% is reahzedcomprised of a mixture of liquid and crystalswhich had formed in the catholyte.

Examples III-VII In order to establish that the resulting reactiveinter- Various ot-haloxylenes, which can be obtained either m i yp-xylyl ne, that is, te rachloro-p-xylylene, was commercially or whichcan be prepared in accordance present in the h ly a P r i n Of t e Cystals were with well-known techniques were electrolyticallypolymisolated y filtering the catholyte miXhlfe at about erized asdescribed in Example I above. In each instance, C. These crystals werethen reacted with bromine and platinum was employed for the anode andmercury for there was obtained therefromot,ot'-dibromo-ot,ot,ot',ot'-tetrathe cathode. Identification of theresulting polymers was chloro-p-xylylene. Identification of theresultant product established by infrared spectrum analysis andelemental was made by infrared spectrum analysis and by a cornanalysisin the same manner as that set forth in Examparison of its melting pointwith :a known sample of the ple I. The results of these examples andtheir respective same compound. control conditions are set forth inTable I below: The remainder of the catholyte mixture was dissolvedTABLE I Cathode Anode Current Current Example ot-haloxylene StartingMaterial potential cathode (milliefficiency Polymer obtained (volts)voltage amperes) (percent) 111 a,ot-dichloro-p-xylene 1.2 3. 03.5 28-5435 Poly(p-xylylene). IV a,ot,a,a,tz,a-hexaehl0ro-p-xylene 0.7 3.06.040-110 95 Poly(d,ot,tt,ot,-tetrachloro-p-xylylene). Voi,a,-2-trichloro-p-xylene -1.4 3. 44.0 26-65 Poly(2-chlorop-xy1ylene).VI a,a',-dibromo-p-xylene 1.2 2.5-4.5 25-69 90 Poly(p-xylylene).

- 1.2 2. 34.2 20-70 90 Poly(2-chloro-p-xylylene).

As is well known to those skilled in the art, conversion of p-xylenes tothe corresponding poly(p-xylylenes) involves the formation of reactiveintermediary p-xylylenes. Hence, when an tt-haloxylene compound, havingthe general structure xmoQwmx wherein X, Y, R and z are the same as hasbeen set forth hereinab ove, is converted to the correspondingpoly(pxylylene) in accordance with the present process, it is attendedwith the formation of reactive intermediary p-xylylenes having thegeneral structure in 300 ml. of tetrahydrofuran resulting in a clearsolution. The solution was permitted to stand at room temperature for aperiod of about 5 minutes whereupon a precipitate formed. Theprecipitate was identified by infrared spectrum analysis and elementalanalysis as poly(ot,ot,ot,ot'- tetrachloro-p-xylylene). In order toinsure that polymerization was complete, the precipitate was kept undernitrogen for a period of 14 hours and then isolated from the solvent byfiltration. The precipitate was then washed with tetrahydrofuran anddried. There resulted a yield of poly(ot,at,a,ot'-tetrachloro-p-xylylene) in an amount of 0.1454 g. which wascomputed to be the equivalent of a current efliciency of While theinvention has been described in detail and with particularity, it shouldbe understood that changes, alterations and modifications may be made inthe methods, steps, materials and processes employed herein withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

What is claimed is:

1. An electrolytic process for the preparation of polyp-xylylenes havingthe general structure wherein Y is a member selected from the groupconsisting of hydrogen and halogens; R is an aromatic nuclearsubstitutent group; 2 is an integer from to 4; and n is an integer of atleast about 50, which comprises:

(1) admixing with an electrolyte an a-haloxylene compound having thegeneral structure wherein Y is a member selected from the groupconsisting of hydrogen and halogens; R is an aromatic nuclearsubstituent group; X is a halogen having a bond strength no greater thanthat of Y with the proviso that when Y is fluorine, X is a halogenhaving a lower bond strength than fluorine; and z is an integer from 0to 4;

(2) placing in the electrolyte composition an anode inert to the freehalogens released during electrolysis and a cathode which exhibits anovervoltage sufficient to suppress the formation of hydrogen at thecathode;

(3) supplying an electromotive potential across said anode and cathodewhich results in a cathode potential sufiicient to reduce saida-haloxylene compound; and

(4) recovering a poly-p-xylylene from the cathode.

2. The process of claim 1 wherein the electrolyte consists of :awater-soluble organic solvent and water.

3. The process of claim 1 wherein the anode is selected from the groupconsisting of platinum, platinum metals and carbon.

4. The process of claim 1 wherein the cathode is selected from the groupconsisting of mercury, lead, cadmium, zinc and aluminum.

5. The process of claim 1 wherein the cathode potential is no greaterthan about 2.0 volts.

6. The process of claim 1 wherein the poly-p-xylylene is recovered as acoating on the cathode.

7. The process of claim 6 wherein the cathode is selected from the groupconsisting of mercury, lead, cadmium, zinc and aluminum.

8. The process of claim 2 wherein a compound capable of dissociatinginto ions is added to the electrolyte in an amount of between about 2%6%by volume.

9. The process of claim 8 wherein a concentrated mineral acid is addedto the electrolyte in an amount of between about 2%6% by volume.

10. An electrolytic process for the preparation of polyp-xylylene whichcomprises:

(1) admixing with an electrolyte consisting of water and a water-solubleorganic solvent, an a-haloxylene compound having the general structurewherein Y is a member selected from the group consisting of hydrogen andhalogens; R is an aromatic nuclear substituent group; X is a halogenhaving a bond strength no greater than that of Y with the proviso thatwhen Y is fluorine X is a halogen having a lower bond strength thanfluorine; and z is an integer from O to 4;

(2) placing in the electrolyte composition an anode selected from thegroup consisting of platinum, platinum metals and carbon and a cathodeselected from the group consisting of mercury, lead, cadmium, zinc andaluminum;

(3) supplying an electromotive potential across said anode and saidcathode which results in a cathode potential no greater than about 2.0volts; and

( 4) recovering from the cathode a poly-p-xylylene having the generalstructure YzO- 4Y1}: wherein Y is a member selected from the groupconsisting of hydrogen and halogens; R is anaromatic nuclear substituentgroup; z is an integer from 0 to 4; and n is an integer having a valueof at least about 50. 11. The process of claim 10 wherein the anode isplatinum. A,

12. The process of claim 10 wherein the cathode is mercury.

XY2 -C 2 wherein Y is a member selected from the group consisting ofhydrogen and halogens; R is an aromatic nuclear substituent group; X isa halogen having a bond strength no greater than that of Y with theproviso that when Y is fluorine X is a halogen having a lower bondstrength than fluorine; and z is an integer from 0 to 4;

(2) placing in the electrolyte composition an anode selected from thegroup consisting of platinum, platinum, platinum metals and carbon and acathode selected from the group consisting of mercury, lead, zinc andaluminum;

(3) supplying an electromotive potential across the anode and thecathode which results in a cathode potential no greater than about 2.0volts; and

(4) recovering from the cathode a poly-p-xylylene having the generalstructure lk Y2K? 0Y3}:

wherein Y is a member selected from the group consisting of hydrogen andhalogens; R is an aromatic nuclear substituent group; 2 is an integerfrom 0 to 4; and n is an integer having a value of at least about 50.

16. The process of claim 15 wherein the water-soluble organic solvent isdioxane.

17. The process of claim 16 wherein a compound capable of dissociatinginto ions is added to the electrolyte in an amount of between about 2%6%by volume.

18. The process of claim 17 wherein a concentrated mineral acid is addedto the electrolyte in an amount of 'between about 2%6% by volume.

19. An electrolytic process for the preparation of reactive intermediaryp-xylylenes having the general sructure wherein Y is a member selectedfrom the group consisting of hydrogen and halogens; R is an aromaticnuclear substituent group; and z is an integer from 0. to 4, whichcomprises:

( 1) admixing with an electrolyte composition maintained at atemperature below about C. an a-haloxylene compound having the generalstructure:

wherein Y is a member selected from the group consisting of hydrogen andhalogens; R is an aromatic nuclear substituent group; X is a halogenhaving a bond strength no greater than that of Y with the proviso thatwhen Y is fluorine, X is a halogen having a lower bond strength thanfluorine; and z is an integer from 0 to 4;

(2) placing in the electrolyte composition an anode inert to the freehalogens released during electrolysis, and a cathode which exhibits anove-rvoltage sufficient to suppress the formation of hydrogen at thecathode;

(3) supplying an electromotive potential across said anode and cathodewhich results in a cathode potential suflicient to reduce said ahaloxylene compound; and

(4) recovering from the electrolyte a catholyte which contains thereactive intermediary p-xylyenes in stable form.

20. The process of claim 19 wherein the anode is a member selected fromthe group consisting of platinum. platinum metals and carbon.

21. The process of claim 19 wherein the cathode is a member selectedfrom the group consisting of mercury, lead, cadmium, zinc and aluminum.

22. The process of claim 19 wherein the cathode potential is no greaterthan about 2.0 volts.

23. The process of claim 19 wherein the cathode is isolated from theelectrolyte composition by means of a diaphragm which is inert to theelectrolyte.

24. The process of claim 19 wherein the temperature of the electrolyteduring electrolysis is between about 0 C. to -l0 C.

25. The process of claim 19 wherein the reactive intermediaryp-xylylenes contained in the catholyte are maintained under atemperature of below about 40 C.

26. The process of claim 19 wherein the reactive intermediaryp-xylylenes contained in the catholyte are per mitted to polymerize byraising the temperature of said catholyte to between about 0 C.25 C.

27. The process of claim 19 wherein the reactive intermediaryp-xylylenes contained in the catholyte and having the general structure(Ill):

wherein Y is a member selected from the group consisting of hydrogen andhalogens; R is an aromatic nuclear substituent group; and z is aninteger from 0. to 4 are reacted with a member selected from the groupconsisting of halogens having a bond strength no greater than Y with theproviso that when Y is fluorine, said halogens have a lower bondstrength than fluorine to form a-halop-xylylenes having the generalstructure:

References Cited UNITED STATES PATENTS 2,777,005 1/1957 Errede 260-6683,053,909 10/1962 Davis et al 260-651 3,140,276 7/1964 Forster 204-59 XJOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistwnt Examiner.

1. AN ELECTROLYTIC PROCESS FOR THE PREPARATION OF POLYP-XYLYLENES HAVINGTHE GENERAL STRUCTURE